Value Of CYFRA 21-1, Ca 19-9, Ca-125 and Mutant p53 in Diagnosis of Malignant Pleural Fluids The role of tumor marker determination in the differential diagnosis of pleural effusions has not been established definitely. In this study, to evaluate the contributions of cytokeratin 19 fragment CYFRA 21-1, CA 19-9, CA-125 and mutant form of tumor suppressor gene p53 to the diagnosis of malignant and benign diseases, pleural fluid levels of these parameters were determined quantitatively in malignant fluids (n=20). These values were compared with benign samples (n=10). CYFRA 21-1 concentration was measured by a solid-phase sandwich immunoradiometric assay, while the other parameters were determined by ELISA. In patients with malignant diseases, the mean pleural fluid CYFRA 21-1, CA 19-9 and CA 125 levels were significantly higher than those in patients with benign diseases. While there were no significant differences between the mean mutant p53 levels in malignant and benign pleural fluids, mutant p53 was positive in 65% of malignant fluids and 30% of benign fluids when the detectable mutant p53 levels were considered. For purposes of discrimination between malignancy and benignancy, the highest diagnostic sensitivity, specificity, positive predictivity, negative predictivity and accuracy were provided by CYFRA 21-1 and CA 19-9 (%100), CA 19-9 (% 80), CA 19-9(%90), CYFRA 21-1 and CA 19-9 (%100), and CA 19-9 (%93), respectively. In conclusion, it can be stated that determination of CYFRA 21-1 and CA 19-9 seems valuable as an adjunct to the conventional diagnostic methods for the discrimination between malignant and benign pleural fluids.